This invention relates to a foamed plastic insulator such as an insulated wire and a method for producing the same. More particularly, this invention relates to a type of foamed plastic insulated wire which exhibits high electrical characteristics and high mechanical strength while providing a high degree of expansion in the insulation material. This invention also relates to a method for producing such a foamed plastic insulator such as an insulated wire.
In the field of communication cables, conductors formed with a foamed plastic insulation are widely used. Particularly, foamed polyolefin is commonly used as an insulation layer. Normally, in these cables, the degree of expansion of the foamed plastic insulator is in a wide range for example 10% to 90% under measurement by a specific gravity method.
Generally, foamed plastic insulation is obtained by extrusion method by means of a screw extruder. A chemical blowing agent such as azodicarbonamide, and 4,4'-oxybis(benzenesulfonylhydrazide), or inert gas such as nitrogen, argon, and carbonic acid gases, or gaseous or liquid hydrocarbon of methane, propane, butane, pentane, and hexane, or gaseous or liquidized fluorocarbon such as trichloromonofluoromethane, dichlorodifluoromethan, trichlorotrifluoroethane, and dichlorotetrafluoroethane is usually employed. The plastic material is generally polyolefin such as low density polyethylene, medium density polyethylene, high density polyethylene, polypropylene, and butyl rubber or the mixture of at least two of these materials. Polyethylene and polypropylene are the most ordinary materials used as plastic materials.
A foamed plastic insulator having expansion degree of 20 to 30% is easily produced by using chemical blowing agents of inert gas and any of the plastic materials mentioned above. However, recently, foamed plastic insulators having a high expansion degree such as 50% or more have required in order to enhance electric characteristics and to reduce diameter of the cable. Applicant's various experiments and analysis reveal that elasticity of the plastic materials is the most important factor to obtain a high degree of expansion. Therefore, the swelling ratio which is the index of elasticity of the plastic material is found to be the determinative factor to obtain high degrees of expansion of the insulator.
In order to meet with the above requirement, the applicant has proposed methods for producing foamed plastic insulation having an expansion ratio of 67% or more by using hydrocarbon or fluorocarbon as a blowing agent and plastic materials whose swelling ratio is in a range of 40 to 75%. This is disclosed in Japanese patent application No. 140755/76, now laid open with OPI No. SHO-53-73382. However, other kinds of high foamed plastic insulation formed over a conductor and methods for producing such insulation are still required to fulfill wide ranging requirements. Also, in many applications the use of hydrocarbon or fluorocarbon as the blowing agent is unacceptable. Furthermore in this prior art example nucleating agents must be used.